QEMU/KVM Panic on USB Mount

[Boris Kochergin]

Hans Petter Selasky wrote:
> On Friday 09 July 2010 19:12:59 Sean Bruno wrote:
>   
>> In order to come up with the most convoluted problem possible, I present
>> to you screen shots of a panic from a FreeBSD VM running in QEMU
>> emulation under RedHat's KVM infrastructure.
>>
>> I presented a perfectly functional USB partition from my host machine to
>> the VM.  Then I attempted to mount the partition from FreeBSD and it
>> generated the panics located here:
>>
>> http://people.freebsd.org/~sbruno/usb_panic1.png
>> http://people.freebsd.org/~sbruno/usb_panic2.png
>>
>> This is 100% reproducible.  But not an emergency or anything.
>>     
>
> It might be that there is a conflict that both the VM and the OS is accessing 
> / loading drivers on the same USB device. The panic seems not directly related 
> to USB.
>
> --HPS
>
>   
I followed the code for a while and the cause of the panic appears to be 
that ffs_vgetf() in /usr/src/sys/ufs/ffs/ffs_vfsops.c fails in one of 
the several ways possible, causing in VFS_ROOT() in 
/usr/src/sys/kern/vfs_mount.c to return a non-zero value and resulting 
in the panic. I think the problem calls for someone with serious VFS chops.

The I/O errors from the USB device are suspicious (in that they may 
cause the mount code to misbehave if they occur during its course).

-Boris